The world salt production has crossed two hundred million tons per annum. About 60% of the salt produced is used for industrial applications, chlor-alkali and soda ash industries being the major consumers. Superior quality industrial grade salt is preferred by these industries as the use of such salt reduces the brine purification cost and effluent generation. 40% salt goes for human consumption where improved whiteness has greater customer appeal and can also enhance the stability of iodizing agent in iodized salt.
Solar salt is produced using sea brine, sub-soil brine and lake brine. Salt produced from such brines is invariably contaminated with impurities such as Ca2+, Mg2+, SO42− and heavy metals. Moreover, the salt tends to be less white than desirable. It is therefore of great importance to devise means of making purer solar salt in cost-effective manner.
Reference may be made to the paper entitled “Primary Brine Treatment Operations” by D. Elliott at the 1999 Eltech Chlorine/Chlorate Seminar on Technology Bridge to the New Millenium, Ohio, 13 Sep. 1999, wherein the critical importance of salt purity and the deleterious effect of various contaminants including heavy metals on chlor-alkali manufacture is highlighted.
Reference may be made to the research article: “Rain Washing of Common Salt Heaps” by M. P. Bhatt et al. (Salt Research and Industry 10 (2), 1974, p 13) wherein it is reported that sea salt, as produced in solar pans contains 0.16-18% Ca, 0.3-0.4% Mg and 0.70% SO4, whereas after rain washing the salt contains 0.21% Ca, 0.06% Mg and 0.60% SO4. Although rain washing reduces Mg impurities, the Ca and SO4 impurities cannot be reduced from the harvested salt even by repeated washings. On the contrary, it is observed that the concentration of Ca increases after rain-washing.
In the article “Washing of Strip Mined Rock and Solar Salt at Leslie Salt Corporation US” (Symposium on Salt-I, Vol. 1, the Northern Ohio Geological society Incorporation, Cleveland (1961), p 449-464), A Woodhill has reported that Ca, Mg and SO4 impurities in solar salt can be reduced by mechanical washing. The main disadvantage of the method is that there is a 15-20% loss of salt and the method requires high capital investment. Moreover, the maximum level of reduction of Ca is 70% and embedded impurities are difficult to remove.
In the article “Manufacture of Solar Salt by Series Feeding System” by R. B. Bhatt et al. (Salt Research and Industry, 11, 1979, p 9) it has been reported that solar salt With less impurities of Ca can be produced from sea water by a series feeding method Wherein the salt is harvested is two stages i.e. between 25.5-27° Be′ and 27-29° Be′. Salt harvested in the first stage is of a superior quality. Although this is a good process the drawback is that calcium and sulphate impurities cannot be reduced beyond a point even though higher levels of reduction are desirable.
A. U. Hamidani and J. R. Sanghavi in their paper entitled “Improvement in quality of salt from in-land brine of Kharaghoda area India” (Research and Industry, Vol. 37, March 1992, pp 46-48), have explained a method of reducing the Ca content in salt by establishing a common ion effect in the saturated brine by increasing the sulfate content through addition of either MgSO4 or Na2SO4. The drawbacks of the method are that though the Ca content of salt is reduced, the Mg and SO4 content cannot be reduced. Moreover, it involves compositional changes which are many times difficult from a logistics and cost point of view.
H. M. Patel, in his research article that appeared in the Proceedings of 6th International Symposium on Salt, Vol. 2 pp. 515-533, has disclosed that Ca and SO4 impurities in salt can be reduced using the difference in dissolution rate of NaCl and CaSO4. The main drawbacks of the process are that it employs unit operations like dissolver and chemical process reactor. It also requires addition of lime and soda for the removal of Mg and Ca and subsequent filtration of brine.
In the Indian Patent No. 191912 (notified in the Indian Gazette) entitled “Preparation of Sodium Chloride Containing Low Ca Impurity from Sea Brine in Solar Salt Work” by J. R. Sanghavi et al. it is claimed that addition of a polysaccharide additive namely starch in concentration of 50-150 ppm into concentrated brine can reduce calcium impurity in salt to less than 0.05-0.1 percent as Ca2+. The drawbacks of the process are that it requires addition of hot solution of starch which is both cumbersome and costly, addition has to be repeated several times and no mention is made of the effect of the treatment on other impurities in salt. No explanation is also provided for the origin of the observed effect.
In the U.S. patent (Igo. U.S. Pat. No. 6,812,011 dated 2 Nov. 2004) entitled “An Improved Process for the Removal of Ca ions from the Brine by Marine Cyanobacteria” by S. Mishra et al. it has been claimed that common salt with reduced Ca impurity can be produced from sea/subsoil brine by mopping up Ca in the brine through certain types of marine cyanobacteria. The drawback of this process is that although the process has been demonstrated in small solar pans, it is not readily amenable to scale up.
In the U.S. patent (U.S. Pat. No. 6,776,972 dated 17 Aug. 2004) entitled “A Process for Recovery of Common Salt and Marine Chemicals from Brine in Integrated Manner” by R. N. Vohra et al. it is claimed that common salt and marine chemicals of high purity can be recovered in an integrated manner by forced desulphatation of brine with inexpensive sources of CaCl2 such as distiller waste of Solvay Process prior to crystallization of salt. The process works well for any kind of brine and can also be carried out at large scale but the main drawback is the lack of availability of such calcium chloride source in the vicinity of most salt works. Another drawback of the process is that care must be taken to ensure that fresh brine does not mix inadvertently with desulphated brine in the crystallizer since the excess calcium chloride can form gypsum in the crystallizer that would deteriorate the quality of salt.
Mention may be made to the same patent above wherein it is stated that sub-soil brine such as that available in the Gujarat State of India invariably gives salt that is considerably inferior to that obtained from sea brine, having as much as 0.30-0.40% Ca.
In the patent application GB 20020028351 20021205 dated 9 Jun. 2004 entitled “Extracting Sodium Chloride From Sea Water Using Nano Filtration” by Kenny Conor et al. it is reported that sea water is pretreated to make it suitable for nanofiltration and the nanofiltered sea water is sent to a thermal desalination plant which operates as a sodium chloride concentrator and a distilled water producer. Sodium chloride is crystallized from the concentrated solution and the process provides a high purity sodium chloride suitable for many industries. It is claimed that the salt so produced eliminates many of the requirements of the primary and secondary brine treatment for the chlor-alkali industries. The nanofiltration process has a higher rejection rate for calcium, magnesium and sulfate ions as compared to Na or Cl ions. The drawbacks of this process are that it would entail high capital investment and additional unit operations which would be uneconomical for standalone production of common salt in solar salt works. Moreover, it needs to be noted that whereas NaCl solubility in water is ca. 35%, its solubility in brine is only 25% which means that advantage can be taken of the common ion effect to reduce Ned solubility in brine which advantage would be lost if the divalent ions were to be completely removed by the process of nanofiltration and more time would be required for evaporation.
In the patent application GB 19540033194 19541116 dated 19 Dec. 1936 entitled “Improved Method of Preparing Sodium Chloride Brines of High Purity” Albright and Wilson have claimed that sodium chloride brines low in calcium sulfate content are prepared by dissolving solid sodium chloride contaminated with calcium sulfate in water in presence of a polyphosphate soluble in brine in the concentration range of 50-100 ppm. It is claimed that the amount of calcium sulfate is further decreased by dissolving solid sodium chloride in the presence of both the poly phosphate and water-soluble alkaline earth metal compound such as calcium chloride or acetate or barium chlorides up to 1% level. The drawbacks of this process are that it is less appropriate for solar salt production and more appropriate as a means of post-treatment of brine obtained by dissolving salt.
In their patent application (U.S. Pat. No. 3,891,297 dated 24 Jun. 1975) entitled “Crystallization of sodium chloride of reduced calcium sulfate content in presence of about 5 to about 500 ppm” by H. W. Fiedelmart a process for the preparation of the cubic crystalline form of sodium chloride is described either by (1) a feed and bleed procedure comprising admixing an alkali metal phosphate with an aqueous solution of salt to increase the super saturation of calcium sulfate there in and evaporating the brine at an elevated temperature and reduced pressure to cause crystalization of pure salt and concomitantly bleeding brine from the chamber to the feed brine such as to maintain the calcium sulfate in the dissolved state and prevent its precipitation with salt or by (2) subjecting the brine to solar evaporation to concentrate the same to the salt point, i.e. that point at which the salt will crystallize from the brine, adding an alkali metal polyphosphate to brine at this point to increase the super saturation of calcium sulfate there is and processing the brine for salt production following the conventional method. The process involves addition of costly chemicals at a very high dosage level.
In their patent application (WO 2004069371 dated 19 Aug. 2004), Kamishima Hiroshi et al. have claimed that sodium chloride crystals with reduced impurities can be produced from aqueous sodium chloride solutions by passing the solution through a column packed with an adsorbent on to which the impurity is selectively adsorbed. The method also provides a sodium chloride composition for preparing artificial seawater for use in algae cultivation, which is reduced in Mg ion or Ca ion concentration. The drawbacks of the process are that it is not applicable to a multi-component system like sea/sub-soil brine. This process does not give any clue about the production of superior quality salt directly from sea/sub-soil brine in a solar salt works.
In the patent (U.S. Pat. No. 4,072,472 dated 7 Feb. 1978) on High purity salt from high sulphate salt deposits by A. Lukes Jerome it is reported that subterranean salt deposit is solution mined, and the resulting calcium and sulfate contaminated brine is treated with soda ash to precipitate calcium compounds. After settling the slurry the clear brine is evaporated in a series of solar ponds to produce high-grade sodium chloride. This process is not economically feasible for large solar salt works where salt is produced from sea/sub-soil brines. Moreover, the process removes only calcium content from salt.
It is well known that the suspended matter in surface water, effluents, wastewater, liquid waste and water from various other sources is removed by sedimentation technique. It is to further known that coagulating agents such as iron salts or aluminium salts have long been employed to improve the conditions for sedimentation. Alum (Al2(SO4)3.18H2O) being a very cheap source of aluthinium sulfate, is widely used as a coagulating agent for the above purpose (K. Dentel and J. M. Gosset, J. Am. Water Works Assoc. April 1988, p 187-188). Sulfate ion in alum appears to act as a catalyst in the formation of polynuclear complexes and their linkage to form a solid lattice (A. C. Venneulen et al. J. Colloid Interface Sci. 57, p 115 (1976)). M/s Tramfloc, Inc., Tempe, Ariz., USA has come out with commercial polyacrylamide based synthetic flocculants for clarifying wastewaters and brines.
In the U.S. Pat. No. 3,647,396 dated 7 Mar. 1972 entitled “Production of High Purity Salt”, H. W. Dewittie et al. have claimed to have developed a process for the recrystallization of sodium chloride in the form of high purity cubic crystals from a sodium chloride source containing calcium sulfate impurity by multieffect evaporation preceded by treatment of the hot sodium chloride saturated brine by flocculants and settling, to cause the undissolved calcium sulfate particles and other suspended solids to agglomerate and settle out of the brine prior to recrystallization of sodium chloride eliminating the conventional requirement for filtering the hot brine. The main drawbacks of the process are that it involves recrystallization which is expensive, time consuming and energy intensive. There is no mention of the utility of the method for production of pure salt directly from sea brine in solar salt works.
AJK Environmental Specialties, Inc., Marchant Ville, N.J., USA has also come out with a commercial flocculating agent by the name of Aquasorb which is a cross linked polymer comprising solely of sodium polyacrylates. It is claimed that the product works well for brine clarification and removal of Ca and Mg. Qumi International, Inc., Texas, USA has also claimed to have produced such polyacrylamide-based flocculants and coagulants for similar purposes. These flocculating agents are used to reduce residual Ca and Mg impurities in brine, such brine being thereafter used directly in industrial applications such as in chlor-alkali and soda ash industries.
It will be evident from the prior art that the purity of salt obtained in solar salt production is indicated to be largely influenced by the composition of the brine although proper methods of brine management such as care to charge and discharge crystallizers at appropriate density, fractional crystallization through series feeding, and deep charging can improve the quality of the salt. The prior art has also provided the limits of solar salt purity achievable in the field through the above Means. It will be further evident that salt quality has been reported to be improved by forcibly altering the composition of brine such as by passing through nanofiltration membrane to reduce divalent ions, adding polyphosphates to maintain gypsum in a supersaturated state, treating with marine cyanobacteria, treatment with starch, and addition of soluble inorganic salts such as calcium chloride, magnesium sulphate or sodium sulphate to forcibly precipitate calcium sulphate prior to salt production. Further, several methods have been described for post purification of salt/brine including: mechanical washing, recrystallisation, treatment with flocculants, selective removal of divalent ions by nanofiltration, and chemical methods of purification, which are typically adopted by downstream industries and not during solar salt manufacture. Whereas use of alum and other flocculation aids for purification of field harvested salt is reported as indicated above, no mention is made in the prior art of any attempt to use alum in solar salt production directly, whereby salt crystallizes during solar evaporation in purer form as a result of events brought about by the clarification of brine prior to charging into the crystallizer, and whereby the salt produced from such alum-treated brine is further refined by the cost-effective methods of water/rain washing of the salt heaps in solar salt works itself to have common salt of a purity hitherto not reported for any solar salt, particularly purity with respect to calcium and sulphate impurity levels. Nor is there any mention in the prior art of changes in crystal morphology brought about by improved clarity of brine during solar salt production and its effect on salt purity.